When Does a Geyser Occur?
A geyser occurs when a
hot spring erupts, hurling a column of water and steam high into the air. These
springs are situated in regions which were formerly volcanic and which have
retained considerable heat near the surface. They usually have craters with
well-like shafts penetrating into the earth. The water which gathers deep down
in these shafts becomes heated until the lower part is changed into steam.
The pressure of the steam steadily mounts to a point when it suddenly hurls the water above it into the air, sometimes to a height of over 100 feet. The chief geyser districts are in Iceland (home of the Great Geyser), in the Yellowstone National Park, Wyoming, United States, and in New Zealand. For four years Waimangu in New Zealand, the greatest of all geysers, was capable of spouting jets up to 1,500 feet.
Geysers are temporary geological features. As the water boils, the resulting pressure forces a superheated column of steam and water to the surface through the geyser’s internal plumbing. The formation of geysers specifically requires the combination of three geologic conditions that are usually found in volcanic terrain.
The heat needed for geyser formation comes from magma that needs to be close to the surface of the earth. In order for the heated water to form a geyser, a plumbing system made of fractures, fissures, porous spaces, and sometimes cavities is required. This includes a reservoir to hold the water while it is being heated. Geysers are generally aligned along faults.
There are two types of geysers: fountain geysers which erupt from pools of water, typically in a series of intense, even violent, bursts; and cone geysers which erupt from cones or mounds of siliceous sinter (including geyserite), usually in steady jets that last anywhere from a few seconds to several minutes. Old Faithful, perhaps the best-known geyser at Yellowstone National Park, is an example of a cone geyser. Grand Geyser, the tallest predictable geyser on earth, (although Geysir in Iceland is taller, it is not predictable), also at Yellowstone National Park, is an example of a fountain geyser.
There are many volcanic areas in the world that have hot springs, mud pots and fumaroles, but very few have erupting geysers. The main reason for their rarity is because multiple intense transient forces must occur simultaneously for a geyser to exist. For example, even when other necessary conditions exist, if the rock structure is loose, eruptions will erode the channels and rapidly destroy any nascent geysers.
As a result, most geysers form in places where there is volcanic rhyolite rock which dissolves in hot water and forms mineral deposits called siliceous sinter, or geyserite, along the inside of the plumbing systems which are very slender. Over time, these deposits strengthen the channel walls by cementing the rock together tightly, thus enabling the geyser to persist, as mentioned in the previous section.
Geysers are fragile phenomena and if conditions change, they may go dormant or extinct. Many have been destroyed simply by people throwing debris into them while others have ceased to erupt due to dewatering by geothermal power plants. However, the Geysir in Iceland has had periods of activity and dormancy. During its long dormant periods, eruptions were sometimes artificially induced—often on special occasions—by the addition of surfactants to the water.